The present invention relates to a power amplifier more particularly the invention is primarily intended for use when delivering high output power in a frequency range 0 to 100 kHz, i.e. power without upper limit, however often in a range 50 to 2000 W, for driving resistive and reactive loads like loudspeakers, motors and other transducer types.
Related art is known from U.S. Pat. No. 3,808,545, U.S. Pat. No. 4,611,180, U.S. Pat. No. 5,179,352 and GB 1,584,941. Among these, particularly U.S. Pat. No. 5,179,352, however partially also GB 1,584,941, relate to a signal correction technique similar to a technique which is also utilized in embodiments of the present invention. U.S. Pat. No. 3,808,545 relates to a power amplifier which exhibits circuitry having features which are also utilized in embodiments of the power amplifier in accordance with the present invention, with a bridge connection and grounding of the output terminal of the output amplifier stage.
The more usual power amplifier constructions consist of:
An input stage. PA1 A voltage amplifier stage. PA1 A current amplifier stage. PA1 A low supply voltage for all stages up to the current amplifier stage, has the effect that all components may be selected in accordance with their small signal characteristics, such as signal/noise ratio, bandwidth and temperature stability. PA1 A low power dissipation in the same components leads to increased reliability and thermal stability. PA1 A lower distortion is achieved, since there are no transistors with a large voltage swing in the stages up to the current amplifier stage. (These will result in distortion due to voltage dependent capacitances.) PA1 Fewer components result in a lower error rate and improved reliability. PA1 It is possible to use a standardized solution: All stages up to the current amplifier stage are the same, independent of the output power, and this simplifies storage and service operations. PA1 In most cases it is possible to ground the cooled electrode of all or half of the output transistors, which simplifies the mounting thereof, lowers the risk of errors and improves the cooling effect.
The task of the input stage is usually to change the operating point of the signals from around ground to around one or both of the supply voltages.
The voltage amplifier stage is intended to increase the signal voltage to a level that can provide a full output from the current amplifier stage.
The current amplifier stage usually has a voltage gain somewhat less than 1, and a current gain that is sufficient to isolate the load from the voltage amplifier stage.
When high power is desirable, one should use a higher supply voltage for the voltage amplifier stage than for the current amplifier stage, to be able to get the highest possible power from the current supply for this stage. (The voltage amplifier stage must be able to drive the current amplifier stage into saturation.) This puts restrictions on the input stage, in which one either has to choose components in accordance with their ability to withstand voltage, instead of e.g. their noise characteristics, or one has to increase complexity by e.g. connecting components in series.
Great demands are also made on the voltage amplifier stage. The transistors in this stage must stand up to the full supply voltage, and since this stage is critical as regards the linearity of the amplifier, such a large current will often run through it, that these transistors will heat up and require local cooling. This may have the effect that the product is thermally more unstable, and it may also have an unfortunate influence on the product lifetime.